1. Field of the Invention
The present invention relates generally to point-to-multipoint telecommunication systems. More particularly, the invention relates to packet-based protocols for bi-directional data interchange on a Passive Optical Network (PON).
2. Background
With the explosion of the Internet content and other multimedia services, telephone line modems are becoming progressively less adequate as means for connecting personal computers to high-speed networks. Several access systems with higher bandwidth capabilities have evolved, including Integrated Services Digital Network (ISDN), Digital Subscriber Line (DSL), and cable modems. These and similar systems are typically bandwidth-limited to rates of the order of several megabits per second. It is desirable to increase the connection rates further to satisfy the growing end-user demand for bandwidth.
Because of its many advantages, optical fiber is widely used in telecommunication systems. The advantages include low signal attenuation, immunity to electromagnetic interference, low crosstalk, fast propagation speed, physical flexibility, small size, low weight, and, most important, high bandwidth. Bringing optical fiber to the end-users is therefore desirable for a number of reasons, including the higher bandwidth that optical fiber would make available to the end-users. But the expense of installing optical fiber in place of the copper wires that typically connect each individual end-user to the central office remains high.
Passive optical network topology provides a good match with the need to connect multiple users to a central point. A passive optical network is an optical transmission system that brings optical fiber to the end-user without requiring individual fiber optic links from each end-user to the central office. Depending on its points of termination, a passive optical network can be called fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), or fiber-to-the-home (FTTH) network. A passive optical network typically includes a central controller, such as an optical line terminal, at the communication company's office, and a plurality of remote network nodes, such as optical network units or customer premises equipment, located near end-users of the network. In a PON, the central controller and the remote network nodes are interconnected by optical fiber and optical splitters/combiners (“splitters” hereinafter).
FIG. 1 illustrates the topology of a representative passive optical network 100. Reference numeral 110 designates an optical line terminal (OLT) that communicates with multiple optical network units (ONUs) over the PON 100. The signals broadcast from the OLT 110 travel through optical fiber spans 140 and passive splitters 120, and are received by the ONUs 130. Each ONU 130-N corresponds to a discrete end-user or application. The signals sent by the ONUs 130 travel in the reverse direction to the OLT 110.
Note that the PON topology differs from the “star” configuration where each end-user is connected to a central location by a dedicated circuit, which may be a dedicated physical or virtual circuit. The topological differences are important for at least two reasons. First, the OLT 110 broadcasts (point-to-multipoint) over PON 100, so that every ONU 130-N receives the broadcasts. Broadcasting raises privacy concerns because each ONU receives not only the transmissions intended for it, but also all other transmissions from the OLT 110. Second, the uplink transmissions (from the ONUs 130-N to the OLT 110) must be multiplexed because the multiple ONUs 130-N must share the total bandwidth available on the shared transmission medium of network span 140-1. Preferably, the available bandwidth should be allocated dynamically and adaptively.
Because of the need to multiplex and the need to secure downlink communications, protocols used in star topologies may not work well in PON topologies. A need therefore exists for a flexible and efficient communication protocol that will provide multiplexing for secure point-to-multipoint communications over a passive optical network topology.